First, a sleep mode operation will be described. While a Mobile Station (MS) communicates with a Base Station (BS) in normal or active mode, if traffic to be transmitted to or received from the BS is not present, the MS transmits an Advanced Air Interface Sleep Request (AAI_SLP-REQ) message to the BS to request transition to a sleep mode. The BS transmits an Advanced Air Interface Sleep Response (AAI_SLP-RSP) message to the MS as a response to the AAO_SLP-REQ message. Upon receiving the AAI_SLP-RSP message, the MS transits to the sleep mode by applying a sleep parameter included in the AAI_SLP-RSP message. The sleep parameter includes a sleep cycle, a listening window, and the like. Alternatively, the BS may cause the MS to transit to the sleep mode by directly transmitting an unsolicited AAI_SLP-RSP message to the MS.
FIG. 1 illustrates operation of an MS in sleep mode in an IEEE 802.16m system. As illustrated in FIG. 1, after transiting to sleep mode from normal mode, an MS operates in sleep mode by applying an initial sleep cycle. As shown, a first sleep cycle includes only a sleep window. Sleep cycles including a second sleep cycle after the first sleep cycle include a listening window and a sleep window. Upon receiving an Advanced Air Interface Traffic Indication (AAI_TRF-IND) message including a negative indication during a listening window of an n-th sleep cycle, the MS determines that there is no traffic transmitted to a downlink and sets the n-th sleep cycle to double an (n−1)-th sleep cycle. Upon receiving an AAI_TRF-IND message including a positive indication during a listening window of an (n+1)-th sleep cycle, the MS sets the (n+1)-th sleep cycle to an initial sleep cycle.
FIG. 2 illustrates a process of receiving an AAI_TRF-IND message at an MS. A BS transmits an AAI_TRF-IND message at least once to a sleep mode MS during a listening window. In the prior art, if the BS transmits an AAI_TRF-IND message including a positive indication, the BS determines, without receiving a response from the MS, that the MS sets a sleep cycle to an initial sleep cycle the moment the BS transmits the AAI_TRF-IND message. However, if the MS does not receive the AAI_TRF-IND message including a positive indication transmitted by the BS, the MS waits for the AAI_TRF-IND message during a listening window. In this case, sleep cycle synchronization between the MS and the BS fails. Accordingly, as illustrated in FIG. 2, if the MS does not receive the AAI_TRF-IND message during a listening window, the MS transmits an AAI_TRF_IND-REQ message to the BS. Upon receiving the AAI_TRF_IND-REQ message, the BS transmits an AAI_TRF_IND-RSP message including a positive or negative indication to the MS. The AAI_TRF-IND message includes a frame number in which the AAI_TRF-IND message is most recently transmitted. Upon receiving the AAI_TRF_IND-RSP message, the MS checks the frame number and a traffic indication to establish broken sleep cycle synchronization between the MS and the BS. FIG. 2 shows the case where the BS transmits a positive indication as a response to the AAI_TRF_IND-REQ message.
In the prior art, upon receiving the AAI_TRF-IND message including a positive indication from the BS, the sleep mode MS sets a sleep cycle to an initial sleep cycle. If there is traffic to be received from the BS even after a listening window, the MS may continue to receive traffic transmitted by the BS by implicitly extending the listening window. However, the listening window is extended within a current sleep cycle. FIG. 3 illustrates extension of a listening window at an MS. As illustrated in FIG. 3, if a BS continues to transmit traffic to an MS, since extension of a listening window is limited within a sleep cycle, the BS should transmit an unnecessary AAI_TRF-IND message. That is, in the case where the BS should successively transmit downlink traffic, a process in which the BS unnecessarily transmits the AAI_TRF-IND message to the MS and the MS sets a sleep cycle to an initial sleep cycle is repeated.
When transmitting the AAI_TRF-IND message to MSs operating in sleep mode, the BS broadcasts one AAI_TRF-IND message to all sleep mode MSs, listening windows of which are overlapped. However, when receiving an AAI_TRF-IND message including a positive indication, sleep mode MSs set a sleep cycle based on a value of a Next Sleep Cycle Flag (NSCF) included in an SLP-REQ message and an SLP-RSP message. When receiving an AAI_TRF-IND message including a negative indication, sleep mode MSs set a sleep cycle to double a previous sleep cycle. If a plurality of sleep mode MSs repeat these processes several times, listening windows of sleep mode MSs may not be overlapped. Accordingly, since the BS should frequently transmit the AAI_TRF-IND message, overhead may be encountered.
In the prior art, when an MS receives the AAI_TRF-IND message including a positive indication, it is not defined when a sleep cycle is set. Namely, it is not defined whether to set a start point of a sleep cycle to a start point of a listening window during which an AAI_TRF-IND message including a positive indication has been received or to a start point of a next sleep cycle of a listening window during which an AAI_TRF-IND message including a positive indication has been received.
As described above, according to the prior art, overhead in that a BS should frequently transmit an AAI_TRF-IND message may occur.